1. Field of the Invention:
This invention relates to a surface-mount network device, and more particularly to a surface-mount electronic component having a plurality of lead terminals and capable of achieving a variety of composite circuits in one component.
2. Description of the Prior Art:
In recent years, with decreasing size of electronic apparatus, development of electronic components that contain a plurality of passive elements such as resistors, capacitors, coils, etc., in one package has been actively under way (such electronic components being hereinafter referred to as network devices). Network devices such as pull-up/pull-down resistor network devices extensively used in digital circuits, RC network devices for suppression of interface noise, and LCR network devices for filters have been making a great contribution to increasing the number of electronic components mounted on one printed circuit board and decreasing the number of packaging processes.
Network devices are used, for example, to provide interface passive element circuits for connection between integrated circuits (ICs) mounted on a printed circuit board (a mounting substrate). FIG. 1 shows an example of such an interface passive element circuit interconnecting two ICs. Block C illustrated in FIG. 1 shows a bussed circuit in which one end of each resistor R1 is connected in parallel to one of the 4-bit bus lines interconnecting the digital ICs while the other end thereof is connected to the power source Vcc. The circuit of block C constitutes a pull-up resistor circuit. Block A shows an isolated resistor network circuit in which one end of each resistor R2 is connected to one of the 4-bit parallel lines interconnecting the digital ICs while the other end thereof is connected in series to one of the 4-bit bus lines interconnecting the digital ICs. Block B shows a capacitor network for suppression of noise in which one of paired electrodes in each capacitor C1 is connected in parallel to one of the 4-bit bus lines interconnecting the digital ICs while the other electrode thereof is grounded.
FIG. 2 shows a prior art single in-line package (SIP) resistor network device which has been used in general purpose applications. The SIP type network device includes an alumina or ceramic insulating substrate 1 on which a circuit is formed, a plurality of lead terminals 5 each gripping the insulating substrate 1 from both sides, and a molding compound 7 encapsulating the insulating substrate 1. A wiring conductor 2, terminal conductors 3 and resistors 4 are disposed on the insulating substrate 1 to form a circuit thereon. The lead terminals 5 each have a clip portion 5a that grips the insulating substrate 1 from both sides, the clip portion 5a being both electrically and mechanically connected to the terminal conductor 3 on the insulating substrate 1 by soldering 6.
The SIP type resistor network device of the above construction is mounted on a printed circuit board in such a way that the insulating substrate 1 is positioned perpendicular to the main surface of the printed circuit board. The lead terminals 5 on the SIP type network device are inserted in through-holes provided in the printed circuit board. On the other hand, a dual in-line package (DIP) type network device is mounted on a printed circuit board in such a way that the insulating substrate contained therein is positioned parallel with the main surface of the printed circuit board. In other words, the DIP type network device is surface-mounted on the printed circuit board. Therefore, the SIP type network device has an advantage over the DIP type one in that the former takes a smaller space on the printed circuit board when mounted thereon. The SIP type network device has the further advantage that since its terminal pitch (lead terminal spacing) is made to match that of commonly used insert type (through-hole type) IC package connectors, it is compatible with the commonly used through-hole type IC packages. Also, since it has a single in-line input terminal configuration, the SIP type network device is best suited for achieving a passive element circuit connected in parallel to bus lines, such as shown in blocks B and C in FIG. 1.
In the SIP type network device, a variety of network circuits can be constructed, such as those shown in FIG. 3A to FIG. 3G, by combining various elements such as resistors and capacitors on the two main surfaces of the insulating substrate. Therefore, the SIP type network device is extensively used in digital interface applications.
However, with the progress of surface-mounting techniques, new IC packages having a smaller terminal pitch than 2.54 mm commonly used on the DIP type have come into use, such IC packages including SOP (small outline package), QFP (quad flat package), PLCC (plastic leaded chip-carrier), and LCC (leadless chip-carrier) types having terminal pitch of 1.27 mm, 1.0 mm, 0.8 mm, 0.65 mm and 0.5 mm, respectively.
On the other hand, the common terminal pitch of the SIP type network device is 2.54 mm or 1.78 mm. It should also be noted that it is extremely difficult to make smaller than 1.27 mm the through-hole pitch on a printed circuit board. This has therefore given rise to a problem in terms of compatibility with the above-mentioned new type IC packages. The SIP type network device is a through-hole type electronic component, but in recent years, increasing numbers of high packaging density printed circuit boards have come to be used on which only surface mountable electronic components are used, such components being surface-mounted on both sides of the printed circuit board. Surface-mount electronic components have therefore come to be used extensively. Of a plurality of electronic components mounted on one printed circuit board, if only SIP type network devices are inserted in through-holes of the printed circuit board while all other electronic components are surface-mounted, there will arise the problem of increased costs of the printed circuit board. This is a shortcoming of the insert type SIP type network device.
FIG. 4A is a plan view of a surface-mount network device in another prior art. FIG. 4B shows a cross section thereof. The network device illustrated is a SOP type resistor network device. As shown in FIG. 4A and FIG. 4B, a terminal conductor 11 and a resistor 12 are formed on an insulating substrate 10, the terminal conductor 11 being connected to a lead terminal 9 by soldering 13. The connection may sometimes be provided by wire bonding or other connecting methods. The insulating substrate 10 is encapsulated in a plastic mold 8. The plastic mold 8 is formed by transfer molding or injection molding. Using such molding methods to form the plastic mold 8 assures production of a package having a high dimensional accuracy and makes it easier to automatically mount network devices on a printed circuit board using a mounter.
The prior art SOP type resistor network device has a dual in-line configuration in which a plurality of lead terminals 9 extend outwardly from the longer sides of the insulating substrate 10, and is mounted on a printed circuit board in such a way that the main surface of the insulating substrate 10 is positioned parallel with the main surface of the printed circuit board. Therefore, this network device is suitable for the isolated resistor network device circuit of block A shown in FIG. 1. Also, in the above described construction of this network device, it is possible to reduce the terminal pitch to less than 1.27 mm.
However, the prior art SOP type network device involves difficulty when it comes to reducing the package width. This is because the main surface of the insulating substrate 10 is positioned parallel with the main surface of the printed circuit board. In the case of the prior art SOP type network device, it is not possible to reduce the package width to less than 3 mm.
If a pull-up circuit such as shown in block C in FIG. 1 is to be formed using a dual in-line SOP type network device, the SOP type network device will have a circuit shown in FIG. 4D. In order to mount a dual in-line SOP type network device having such a circuit, the printed circuit board will have a circuit configuration as shown in FIG. 5. There are mounted on the printed circuit board a digital IC 16, a SOP type resistor network device 14, and an I/O connector 17, an 8-bit bus 18 connecting the digital IC 16 to the SOP type resistor network device 14 and the SOP type resistor network device 14 to the I/O connector 17.
As is apparent from FIG. 5, the wiring pattern of the 8-bit bus lines 18 is complex and takes a large space on the printed circuit board, thus increasing the virtual area needed on the printed circuit board for mounting the network device.
FIG. 6A shows still another example of the prior art. The component shown is a surface-mount multiple chip resistor network device. Semicircular recesses are formed in side surfaces of an insulating substrate 19 along the longitudinal direction thereof, and thick film electrodes are formed on the upper surface of the substrate 19 as well as in the recesses in the side surfaces. The thick film electrodes function as output electrodes 20 for connecting the network device to wiring on an external printed circuit board. A resistor 21 is printed and baked between the output electrodes 20. FIG. 6B shows an equivalent circuit for the multiple chip resistor network device illustrated in FIG. 6A. The output electrodes 20 are connected directly to wiring on the printed circuit board by soldering. Therefore, if there is a difference in thermal expansion coefficient between the insulating substrate 19 and the printed circuit board, cracks are likely to be caused in the soldered connections.
Also, when forming a large number of elements (e.g., resistors) on the insulating substrate 19, it is necessary to extend the length of the insulating substrate 19. An extended length of the insulating substrate 19 will make the insulating substrate 19 more susceptible to breakage when the printed circuit is warped due to mechanical stress. Further, since the output electrodes 20 are soldered directly to the printed circuit board, there is a possibility that the electrodes may be damaged if soldering temperature is too high or soldering time is too long.
Thus, the multiple chip network device has the disadvantage that its soldered connections are less reliable and less resistance to mechanical stress. Also, the multiple chip type network device has a simple construction with a plurality of elements arrayed thereon and, usually, does not have internal wiring. Therefore, when using the multiple chip network device as the pull-up/pull-down resistor circuits illustrated in block C in FIG. 1 or as the noise suppression capacitor network illustrated in block B where common grounding is provided using the bus lines via capacitors, it becomes necessary to provide common connection wiring on the printed circuit board for Vcc, GND, etc. Also, a double-sided wiring technique or a multilayer wiring technique is required to process portions where the wiring lines cross each other. Using these techniques involves the problem of increased production costs of the printed circuit board because of the need for formation of through-holes and multilayer wiring.